Electrode means to electrostatically focus ions separated by a mass spectrometer on a detector



Nov. 9, 1965 R. D. CRAIG 3,217,161

ELECTRODE MEANS TO ELECTROSTATICALLY FOCUS IONS SEPARATED BY A MASS SPECTROMETER ON A DETECTOR Filed Dec. 10, 1962 United States Patent 3 217,161 ELECTRODE IVLEANS TO ELECTROSTATICALLY FOCUS IONS SEPARATED BY A MASS SPEC- TROMETER ON A DETECTOR Robert Derek Craig, Bowdon, England, assignor to Associated Electrical Industries Limited, London, England, a British company Filed Dec. 10, 1962, Ser. No. 243,538 Claims priority, application Great Britain, Dec. 29, 1961, 46,669/ 61 3 Claims. (Cl. 250-413) The present invention relates to mass spectrometers and in particular to ion collecting apparatus used in combination with the magnetic analyser of a mass spectrometer.

In a mass spectrometer the sample to be analysed is ionized in an ion source. The ions so produced are accelerated in an ion gun towards an aperture known as the source slit. This aperture acts as a line source from which ions emanate to form a diverging beam which passes into the analyser. The analyser comprises a magnetic field or a combination of electric and magnetic fields, either in series or in tandem. The fields extend across the direction of movement of the ions, along the length of the source slit. In the analyser ions of different mass/ charge ratio are deflected by different amounts. The dispersed beams of ions of different mass/ charge ratio are subsequently received by the collecting apparatus which conveniently includes a surface covered with a layer of material sensitive to ions. The relative number of ions of each mass/charge ratio from the sample can be determined from subsequent examination of the sensitive layer.

As the ions pass through the magnetic analyser their movement can be indicated relative to three mutually perpendicular directions, the x and y directions being across the magnetic field and the z direction being along the magnetic field. Movement of the ions in the x and y directions is determined by the shape and configuration of the analyser fields so that as the ions pass through the analyser the initial single divergent ion beam is dispersed in the xy plane to form individual beams corresponding to ions of different mass/charge ratio. In addition each of the beams undergoes a focussing effect in this plane so that as the beams approach the position of the collecting apparatus they are converging to form a series of images of the source slit.

Movement of the ions in the z direction is not normally controlled. The line images formed at the collector are therefore longer than the source slit, their length being determined either by the initial 2 divergence in combination with length of the source to collector path, or by some collimating aperture. In either case the intensity per unit area of the beam at the collector is clearly less than the maximum obtainable if a suitable form of zfocussing were used. This is a disadvantage if the mass spectrometer is required to detect very low ion concentrations using a collector incorporating as detector a sensitive surface, such as a photographic plate.

Previous methods of increasing sensitivity by focussing in the z-direction have included placing an electrostatic lens close to the source slit, or between the electric and magnetic analysers, or by shaping the field of the electric analyser. These methods suffer from three basic disadvantages namely:

(l) Little, if any, demagnification of the source slit length can be produced at the collector because the image distance is relatively too great.

(2) The maximum usable aperture is limited by the condition that ions should not strike the pole faces of the magnetic analyser.

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(3) Optimum z-focussing conditions cannot be obtained simultaneously for ions of widely different mass/ charge ratio since the path lengths for these will be very different.

The object of the present invention is to provide an improved spectrometer ion collecting apparatus which tends to avoid this defocussing of the ion beam in the zdirection, and to increase the sensitivity of the apparatus.

According to the present invention a mass spectrometer ion collecting apparatus comprises a collector member one plane surface of which is coated with a layer of material which is sensitive to ions and an ion focussing system extending over said layer but spaced therefrom and adapted to focus in a direction across said layer ions which are moving towards said layer from the magnetic analyser of the mass spectrometer.

The ions are preferably focussed in a direction parallel to the magnetic field of the magnetic analyser.

According to a preferred arrangement the focussing system is formed from a plurality of plates of electrically conducting material adapted to be connected respectively to suitable sources of potential and extending across said layer.

The plates are conveniently stacked in pairs so as to define a plurality of aligned gaps through which the ions are adapted to pass to said layer.

In order that the invention may be more readily understood reference will now be made to the accompanying drawing in which:

FIG. 1 is a side view in section of the magnetic analyser of a mass spectrometer including an ion collecting apparatus embodying the invention; and

FIG. 2 is an end view of a magnetic analyser illustrated in FIG. 1.

With reference to the figures, the magnetic analyser comprises two magnet poles 1, 2 defining a gap 3 across which is formed the magnetic field and through which the beam of ions 4 from an ion source 15 is adapted to pass. The beam of ions is dispersed as described above in the x and y directions and defocussed in the z direction in a direction along the magnetic field. This lateral defocussing of the ions is illustrated in FIG. 2.

The ion collecting apparatus comprises a plate 5 supported on insulating supports 16 and having one surface coated with a layer 6 of material which is sensitive to ions. The layer 6 may be made of a photo-sensitive material for example. The ion collecting apparatus is so located that the ions strike the sensitive layer 6 after having passed through the magnetic field between the magnet poles 1, 2.

An ion focussing system is located between the layer 6 and the magnet poles 1, 2 and this focussing system is formed from a plurality of spaced apart thin plates of electrically conducting material 7, 8, 9, 11, 12 and 13 supported on the supports 16. The plates are arranged in pairs 7 and 8, 9 and 11, 12 and 13 so that gaps are defined between the plates of each pair and the pairs of plates are superimposed so that the gaps are aligned. The plates are electrically insulated from each other and are maintained at suitable potentials by a source 17 in order to form an ion focussing lens. For example the plates 7 and 8 may be maintained at +500 volts, plates 9 and 11 at +10 kv. and 12 and 13 at ground potential.

The focussing system is so located that the ion beam on emerging from the magnetic analyser passes through the focussing system before striking the layer 6. As a result the ion beam is focussed in the z direction parallel to the magnetic field.

The focussing system is not limited to the type of lens described but it will be appreciated that the focussing system must be of such a size as to fit into the narrow 3 space between the plate 5 and the top surface of the magnet poles 1, 2.

By using a focussing system of the type described above a considerably shortened image of the source slit can be formed on the sensitive layer 6, and hence the sensitivity of the collector to the detection of weak ion beams is increased. The introduction of the focussing system may alter slightly the position of the image in the x-y plane but this can easily be compensated for by moving the collecting apparatus relative to the magnetic field.

The focussing system may readily be extended over the full length of the sensitive layer 6 and may be located at a constant optimum distance from this layer, so that ion beams of all mass/charge ratios received by the surface undergo the same optimum focussing in the z direction.

In the arrangement illustrated, the plate 5 is fixed in position. In an alternative arrangement the plate is not held rigidly by the supports 16 so that the plate can be moved transversely relative to the path of the ion beam and different portions of the sensitive layer 6 can be exposed to the ion beam.

What I claim is:

1. For a mass spectrometer including a magnetic analyser in which ions from a sample being analysed are passed through a magnetic field between elongated pole pieces and are deflected in a plane extending perpendicular to said magnetic field, ion collecting apparatus comprising a layer of material sensitive to ions, and an ion focussing system extending over the surface of said sensitive layer in a direction along the length of said pole pieces, said system providing an ion focussing field to focus ions in the direction of the magnetic field between the pole pieces ions travelling towards said sensitive layer from said magnetic analyser.

2. For a mass spectrometer including a magnetic analyser in which ions from a sample being analysed are passed through a magnetic field between elongated pole pieces and are deflected in a plane extending perpendicular to said magnetic field, ion collecting apparatus comprising a layer of material sensitive to ions, a plurality of electrodes extending in the direction of the pole pieces and along and over the surface of said sensitive layer and spaced therefrom, and means for connecting said electrodes respectively to suitable potentials so that said electrodes provide an ion focussing field perpendicular to the magnetic field between the pole pieces so as to focus in the direction of said magnetic field ions travelling towards said sensitive layer from said magnetic analyser.

3. For a mass spectrometer including a magnetic analyser in which ions from a sample being analysed are passed through a magnetic field between elongated pole pieces which define a gap therebetween and are deflected in a plane extending perpendicular to said magnetic field, ion collecting apparatus comprising a layer of material sensitive to ions, a plurality of electrically conducting plates arranged in pairs extending along the elongated pole pieces and in parallel planes over the surface of said layer but spaced therefrom so that the adjacent edges of each pair of plates defines a gap and said gaps are aligned with the gap between the elongated pole pieces and means for connecting said plates respectively to suitable potentials so that said plates provide an ion focussing field to focus in the direction of the magnetic field between the pole pieces ions travelling through said aligned gaps towards said sensitive layer from said magnetic analyser.

References Cited by the Examiner UNITED STATES PATENTS 2,691,107 10/54 Berry 2504l.9 2,837,653 6/58 Craig et a1 250-4l.9 2,887,582 5/59 Craig 250-4l.9

RALPH G. NILSON, Primary Examiner. 

1. FOR A MASS SPECTROMETER INCLUDING A MAGNETIC ANALYSER IN WHICH IONS FROM A SAMPLE BEING ANALYSED ARE PASSED THROUGH A MAGNETIC FIELD BETWEEN ELONGATED POLE PIECES AND ARE DEFLECTED IN A PLANE EXTENDING PERPENDICULAR TO SAID MAGNETIC FIELD, ION COLLECTING APPARATUS COMPRISING A LAYER OF MATERIAL SENSITIVE TO IONS, AND AN ION FOCUSSING SYSTEM EXTENDING OVER THE SURFACE OF SAID SENSITIVE LAYER IN A DIRECTION ALONG THE LENGTH OF SAID POLE PIECES, SYSTEM PROVIDING AN ION FOCUSSING FIELD TO FOCUS IONS IN THE DIRECTION OF THE MAGNETIC FIELD BETWEEN THE POLE PIECES IONS TRAVELLING TOWARDS SAID SENSITIVE LAYER FROM SAID MAGNETIC ANALYSER. 